This invention relates to apparatuses for circumscribing objects and, in particular, to apparatuses for circumscribing pipes to mark an even cut-off line thereabout.
It is often necessary to replace sections of pipe due to wear or damage. This applies to a wide range of pipe sizes from a few inches to several feet in diameter. However it is difficult to cut out the old section of pipe cleanly so that it can be replaced with a new length of pipe which can be welded in place easily by butt welding. The problem is that the existing welds tend to be irregular and distort the remaining ends of the pipe when the old section is removed.
Accordingly, after the old section of pipe is removed, a clean cut is made on the remaining sections of pipe to form circular, perpendicular ends. This allows a new length of pipe to be slipped into position with only a minor gap for butt welding.
The circular ends are first marked by running a scriber about the pipe. Conventionally a length of sheet metal having a straight edge, such as shim stock, is wrapped around each remaining end of the pipe so that the straight edge forms a circular line about the pipe. A circumscribing apparatus is then run around the pipe with the edge of the shim stock forming a guide. The circumscribing apparatus marks a circular line on the pipe at a distance closer to each end of the pipe than the sheet metal. In this way an even circular line can be marked on a distorted end of the pipe a distance from an undistorted portion where the shim stock is located. The circular line thus marked can be cut off evenly with a torch. The distance between the remaining sections of pipe can be measured and a new length of pipe accurately cut for fitting between them.
Different types of circumscribing apparatuses have been developed in the past. For example, U.S. Pat. No. 4,345,379 to Pettingill discloses a device for marking a tube at a pre-determined distance from the tube end.
U.S. Pat. No. 4,393,594 to Hoffman describes a collapsible scriber assembly for checking the radial and lateral run out of a wheel and tire assembly.
U.S. Pat. No. 4,419,828 to Farris describes an apparatus for establishing the junction contour for intersecting pipes.
A pipe fitter's scribing and spacing tool is disclosed in U.S. Pat. No. 4,553,305 to Dearman.
However, prior art devices of this general type suffer a number of significant disadvantages. In many cases they are not capable of circumscribing even circular lines about pipes having a wide range of diameters from a few inches to several feet. In other cases the apparatuses extend outwardly too far from the pipe such that they cannot move completely about the pipe due to interference from adjacent pipes or other objects. In other cases the apparatuses are relatively complex or expensive to produce.